A specific acids

Carboxylic A term describing a specific acidic group (COOH) that contrib- utes cation-exchange ability to some resins. 

Sulfonic A specific acidic group (sosh) on which depends the exchange activity of certain cation adsorbents. 

ALEXANDER D. RYABOV AND TERRENCE J. COLLINS III. Kinetics and Mechanisms of Demetalation of Fem-TAML Activators A. Specific Acid Catalysis... 

Identify water or a specific acid as the least drastic dissolving agent for each of the following ... 

Fig. 7.1. a) Specific acid catalysis (proton catalysis) with acyl cleavage in ester hydrolysis. Pathway a is the common mechanism involving a tetrahedral intermediate. Pathway b is SN1 mechanism observed in the presence of concentrated inorganic acids. Not shown here is a mechanism of alkyl cleavage, which can also be observed in the presence of concentrated inorganic acids, b) Schematic mechanism of general acid catalysis in ester hydrolysis. 

The allylic cation (40), formed in a specific acid-catalysed process, is relatively stable thermodynamically, stable enough towards trapping by nucleophiles that the reaction product obtained is almost invariably the naphthalene elimination product. di-Enediynes (42) are formed regiospecifically when the allylic cation (41) is trapped as shown. The walking of methanol around optically active l-methyl-3-ethylallyl... 

Prior to 1967 acetal hydrolysis had been found to be a specific-acid catalysed reaction with the accepted mechanism [equation (46)] involving fast pre-equilibrium protonation of the acetal by hydronium ion, followed by unimolecular rate-determining decomposition of the protonated intermediate to an alcohol and a resonance stabilized carbonium ion (Cordes, 1967). An A-1 mechanism was supported by an extremely large body of evidence, but it appeared unlikely that such a mechanism could expledn the... 

Hydrogen would be the simplest center element. Indeed, chiral Brpnsted acids have emerged as a new class of organocatalysis over the last few years [3-13]. The field of asymmetric Brpnsted acid catalysis can be divided into general acid catalysis and specific acid catalysis. A general acid activates its substrate (1) via hydrogen bonding (Scheme 2, a), whereas the substrate (1) of a specific acid is activated via protonation (Scheme 2, b). 

The single most crucial piece of information necessary for the interpretation of the behaviour of a specific acid-catalyzed reaction is the concentration of the protonated, reactive species this depends on a knowledge of the basicity of the substrate. Until very recently this information has not been available for carboxylic esters, and consequently most work has necessarily been done using relatively weakly acidic media, where it can be assumed that the concern ration of the protonated species is small. 

The mechanism shown in Scheme 3 envisions an association by hydrogen bonding between the catalyst and the carbonyl compound, followed by rate-determining attack of the nucleophile (HaO) and simultaneous transfer of the proton. The rate of this step will depend on the nature and concentration of HA, and the mechanism is consistent with general catalysis. It should be noted that the reverse process consists of a specific acid plus a general base catalysis. A possible general base catalysis mechanism is shown in Scheme 4. The reverse is a specific base plus a general acid catalysis. 

We see here that the mechanism with a pre-equilibrium proton transfer leads to a specific acid catalysis rate law whereas that with a rate-determining proton transfer leads to general acid catalysis. It follows that, according to which catalytic rate law is observed, one of these two mechanisms maybe excluded from further consideration. Occasionally, however, different mechanisms lead to the same rate law and are described as kinetically equivalent (see Chapters 4 and 11) and cannot be distinguished quite so easily. 

There are numerous inherited disorders of lysosomal metabolism in humans. These disorders result from the lack of a specific acid hydrolase and have several clinical manifestations. A variety of substances may accumulate that interfere with normal cell functions, as is the case with the lipidoses (Chapter 9) or mucopolysaccharides (glycosaminoglycans) in the Hurler s disease (gargoylism). 

What about the regioselectivity The obvious explanation is that a cation is formed from die epoxide in a specific acid-catalysed ring opening. But why should the nitrile attack the bottom face of the cation We should expect it to attack the top face preferentially as die hydroxyl group partly blocks the bottom face. 

In the case of apparent general acid catalysis of acetylimidazole hydrolysis, the mechanism can be defined as a specific acid-general base process by comparison with the general base catalysis of N-methyl,N -acetylimidazolium ion. The rate of disappearance of N-methyl,N -acetylimidazolium ion in water at 25° is proportional to the concentration of the basic form of buffer components such as acetate, phosphate, N-methylimidazole, etc., (equation 30) (Wolfenden and Jencks, 1961). The buffer terms show a 1 1 correlation with the general acid-catalyzed rate of acetylimidazole disappearance (Jencks and Carriuolo, 1959) in water at 25°, when the rate expression for the latter reaction is written in terms of equation (32) rather than equation (31), that is, in terms of a general base-catalyzed hydration of protonated acetylimidazole (pX= 3-6). 

The logarithmic concentration diagram applies only for a specific acid and for a particular initial concentration of acid. Such diagrams can be readily obtained from the distribution diagrams previously discussed. The details of... 

You know that you can write an equation for the ionization of a specific acid. However, it is sometimes handy to represent the formation of hydronium ions when acids dissolve in water by a general equation. In this general equation, any monoprotic acid is represented by the general formula HA. Compare this general equation to the specific equation for the ionization of HCl. 

Because both dihydrogen phosphate and hydrogen carbonate (and other substances like them) can be either Bronsted-Lowry acids or bases, they cannot be described as a Bronsted-Lowry acid or base except with reference to a specific acid-base reaction. For this reason, the Arrhenius definitions of acids and bases are the ones used to categorize isolated substances on the stockroom shelf A substance generates either hydronium ions, hydroxide ions, or neither when added to water, so it is always either an acid, a base, or neutral in the Arrhenius sense. Hydrogen carbonate is an Arrhenius base because it yields hydroxide ions when added to water. Dihydrogen phosphate is an Arrhenius acid because it generates hydronium ions when added to water. 

Specific-acid and general-acid catalysts speed up a reaction in the same way— by donating a proton in order to make bond making and bond breaking easier. The two types of acid catalysis differ only in the extent to which the proton is transferred in the transition state of the slow step of the reaction. In a specific-acid-catalyzed reaction, the transition state has a fully transferred proton, whereas in a general-acid-catalyzed reaction, the transition state has a partially transferred proton (Figure 24.3). 

A specific-acid catalyst must be an acid that is strong enough to protonate the reactant fully before the slow step begins. A general-acid catalyst can be a weaker acid because it only partially transfers a proton in the transition state of the slow step. 

Fig. 10.28. Formation of a cytokeratin filament. The central rod of the keratin monomer is principally a-helical structure. A specific acidic keratin monomer combines with a specific basic keratin monomer to form a heterodimer coil (a coiled coil structure). Two dimers combine in antiparallel fashion to form a tetramer, and the tetramers combine head-to-tail to form pro to filaments. Approximately eight protofilaments combine to form a filament. The filament is thicker than actin filaments (called thin filaments or micro filaments) and thinner than microtubules (thick tubes) and is therefore called an intermediate filament.

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